1. Field of the Invention
The present invention relates to a process for making a zinc pyrithione product having an acceptable white or off-white color and containing substantially no undesired 2-hydroxypyridine-N-oxide or metal salt complexes thereof.
2. Description of the Prior Art
Zinc pyrithione [also known as zinc pyridine-2-thiol-N-oxide or bis[1-hydroxy-2(H) pyridinethionato]-zinc] is an excellent biocide. It has been employed as a broad-spectrum anti-microbial agent and preservative in metalworking fluids, plastics, and cosmetics. Its principal use is as an antidandruff agent in hair products. Sodium pyrithione [also called the sodium salt of 1-hydroxy-2-pyridinethione, sodium pyridine-2-thiol-N-oxide, or 2-pyridinethiol-1-oxide, Na salt] is also employed as a preservative in various applications (e.g., metalworking fluids).
The generally accepted route for making these compounds is by the mercaptization of a 2-halopyridine-N-oxide with an alkali metal sulfide or hydrosulfide and a base to make an alkali metal pyrithione and then converting this alkali metal salt to zinc pyrithione by reaction with a zinc salt (e.g., ZnCl.sub.2 or ZnSO.sub.4).
One preferred method for making sodium pyrithione by this route is disclosed in Japanese Pat. No. 1,051,254 which was filed by Pivawer, Schiessl, and Shermer on Apr. 19, 1977, and issued on June 25, 1981. Their method prepared sodium pyrithione by reacting 2-chloropyridine-N-oxide with a substantially equimolar amount or a slight molar excess of NaSH (i.e., from 1 to 1.25 moles per mole of N-oxide). A critical parameter of their process was controlling the pH of the reaction in the range from 7.5 to 11.0. They taught control of the pH could be carried out by adding NaOH simultaneously or concurrently with the NaSH at regulated feed rates.
It should be noted that this method disclosed by Pivawer et al. in their specific examples employed an NaSH and NaOH addition temperature of not less than 75.degree. C. Furthermore, this Japanese patent teaches that alkali metal carbonates may be substituted for NaOH as pH control agents; however, none of the specific examples employed an alkali metal carbonate. In all, no known publication has ever taught the advantages associated with the use of sodium carbonate (e.g., higher yields and a whiter product) as a base when making sodium or zinc pyrithione.
Several problems arise when using this process of Pivawer et al. on large-scale commercial production runs. First, because NaOH and NaSH are highly corrosive, it is difficult to obtain accurate pH readings or keep pH meters operating reliably. Also, the amount of the by-product 2-hydroxypyridine-N-oxide produced will increase when NaOH is used solely as a base in the formation of sodium pyrithione. Formation of this by-product decreases the yields of both sodium and zinc pyrithione and causes impure products to be made.
Besides these difficulties, it should be noted that sodium pyrithione and zinc pyrithione occasionally have problems meeting strict color specifications set by formulators of cosmetics and toiletries. Since the esthetics of cosmetics and toiletries normally require certain desirable colors and the formulators of such products go to great lengths to achieve specific color effects, any ingredient which varies very much from white or colorless may make the colorant formulators' task very difficult. In the cases of sodium and zinc pyrithione, it is believed that unacceptable discoloration results from the presence of unwanted traces of contaminants during the making of the sodium pyrithione. One method of removing these contaminants is to carry out multi-step purification processes. This is costly and adds extra processing steps.
Accordingly, there is a need in the art to overcome the above-stated problems associated with the Pivawer et al. process. Also, there is a need for a better method of preventing or removing unacceptable discoloration of sodium or zinc pyrithione. It is believed that the present invention which involves the making of sodium pyrithione by reacting a 2-halopyridine-N-oxide with Na.sub.2 CO.sub.3 (or a mixture of NaOH and Na.sub.2 CO.sub.3) and NaSH under selected addition temperatures and reaction temperatures and in selected mol ratios meets these needs.